Silicon carbide (SiC) has shown great potential for harsh environment sensor applications. Unlike the electronic devices which typically use a stable and inert structure, SiC sensors include a sensitive sensing surface to respond to gases reaching the sensor surface and an inert contact pad for wire bonding. Stable operation of SiC-based Schottky diodes use a noble metal(s) on top of a noble metal oxide which are on top of a silicon carbide to form a diode. For example, a structure can be palladium on top of palladium oxide which is on top of silicon carbide (a Pd/PdOx/SiC structure). Other structures can be platinum on top of palladium oxide which is on top of silicon carbide (a Pt/PdOx/SiC structure) or platinum on top of platinum oxide on top of silicon carbide (a Pt/PtOx/SiC structure). The structure facilitates detection of hydrogen, hydrocarbons, and other gases at elevated temperatures as high as 600C°. The Schottky diodes use the noble metal oxide as a barrier layer between a noble metal and the substrate. For example, as discussed above, the noble metal oxides can be palladium oxides (PdOx) or platinum oxides (PtOx). The noble metal can be palladium (Pd) or platinum (Pt) or their alloys. The substrate can be silicon carbide (SiC). The noble metal oxide barrier layer prevents silicide-forming reactions between the precious metal and the substrate layers. Sensor testing has shown that a Pd/PdOx/SiC structure and a Pt/PdOx/SiC structure provide stable sensing of hydrogen and hydrocarbons at high temperatures, while operating over a wide range of temperatures. For example, such a diode sensor was tested at 450C° for nearly 1500 hrs. Detection of hydrogen from room temperature to 500C° was also achieved. Further it has been shown the diode sensor can measure hydrogen to the level of at least 250 parts per billion.
While having a sensor element operational at high temperatures is beneficial to make successful measurements, the sensors should be electronically connected to other devices that can record, display, or analyze the measurements. Previously, wire bonding was made directly on the diode surface so that the sensor could be connected to a device. However, the direct wire bonding compromises the integrity of the sensor surface, particularly for long term and high temperature sensing and results in changes in the sensing surface and/or deterioration of the sensing surface. Further, repeatable sensor performance from one sensor to the other is compromised.
In previously constructed sensors, the electrical connection is susceptible to defects in the substrate that affects the readout. Furthermore, an electrical connection made directly to the diode compromises the durability of the sensor. The electrical connection can also physically damage the sensor during operation in certain circumstances.